Automotive alternators that use Lundell rotors have been used in automobiles for decades. Loads from electrical equipment that is mounted onto automobiles due to environmental issues have been increasing rapidly in recent years, and further increases in generated power are being sought from Lundell rotors.
If attempts are made to answer these demands within the scope of conventional design, the alternators are invariably increased in size. Increases in alternator size are undesirable since the weight of and space occupied by such alternators is increased. Increases in alternator size are also known to give rise to new problems such as leading to increased rotor inertia, and engine speed fluctuations and alternator inertial torque interacting and leading to belt vibration and slippage.
Consequently, there is demand to increase alternator capacity while maintaining alternator main body size in its present state.
Conventionally, means of disposing permanent magnets between claw-shaped magnetic poles that face each other in a Lundell rotor have been adopted in order to solve such problems (see Patent Literature 1 and 2, for example).
In addition, examples of magnet mounting methods include: methods in which U-shaped magnets are held on claw-shaped magnetic poles by being fitted onto claw tips of the claw-shaped magnetic poles (see Patent Literature 3, for example), or methods in which a ring-shaped magnet is disposed on an outer circumferential portion of a cylindrical field coil that has been wound onto a boss portion of a rotor core, and the magnet is held by claw-shaped magnetic poles (see Patent Literature 4, for example).
Thus, various methods for holding permanent magnets have been proposed in conventional automotive alternators, but for these permanent magnet holding methods to be of practical use, it is necessary to: (1) increase permanent magnet holding reliability; (2) suppress induced voltages during no-load de-energization; and (3) avoid thermal demagnetization of the magnets by high-frequency magnetic fields that are induced by stator slots.
Each of these factors will now be explained.
(1) Permanent Magnet Holding Strength
In automotive alternators, rotors rotate at high speeds in a vicinity of up to 18,000 to 20,000 rpm when driven by torque that is transmitted from an engine by means of belts and pulleys. Because of this, even if small magnets that weight only a few grams per pole are installed, extremely large centrifugal forces that exceed several tens of kilogram force act on the magnets.
Conventional magnet holding methods have attempted to hold the centrifugal forces that act on the magnets using the claw-shaped magnetic poles themselves. In these magnet holding methods, it is necessary to finish surfaces to be joined of both the magnets and the claw portions with extremely high precision so as to place the two in a state of surface contact. In other words, if the two are placed in point contact, local stresses may be concentrated on the magnets, and the magnets may be damaged. Because raising magnet processing precision is difficult in mass-produced products, it is also possible to consider means for ensuring external shape precision of the magnets using SUS plates or resin molds instead, but these lead to enormous costs.
To facilitate installation of field coils, pole cores are divided axially into two combining sections, and it is also necessary to increase combining precision. Realistically, ensuring such parts precision increases costs significantly during mass production of rotors.
In addition, even if static shape precision is adapted in this manner, magnet holding in automotive alternators is still difficult.
Specifically, since automotive alternators are disposed in engine compartments, they may be placed in high-temperature environments that are several tens of degrees above one hundred degrees Celsius, generating displacements of several tens of μm due to thermal expansion or contraction.
Large centrifugal forces also act on the claw-shaped magnetic poles even when not holding magnets, and the claw tip portions expand approximately 50 to 100 μm radially outward. Thus, the claw-shaped magnetic poles are displaced so as to flap with increases and decreases in engine rotational speed. Since the claw-shaped magnetic poles have a cantilever beam construction, displacement is greater at tip end portions, smaller at claw root end portions, and distances between adjacent claw-shaped magnetic poles also change.
Consequently, if attempts are made to hold the magnets using uniform surfaces despite the presence of such dynamic thermal and centrifugal displacements of the claw-shaped magnetic poles, a great deal of adaptation is required in the magnet holding construction.
Because magnet main bodies or covers that protect the magnets slide and abrade due to displacement of the claw-shaped magnetic poles, it is necessary to ensure reliability of strength for a long time.
Because of these facts, the current situation is such that much further adaptation is required in order to resist the centrifugal forces that act on the magnets and hold the magnets on the claw-shaped magnetic poles, and it is desirable that the magnets be held somewhere other than by the claw-shaped magnetic poles. Thus, in order to avoid the effects on magnet holding of relative displacement between the magnets and the claw-shaped magnetic poles, a conventional improved magnet holding construction has been proposed in which magnets that are magnetized radially are disposed on an outer circumferential side of a yoke portion on axial end portions of a Lundell pole core (see Patent Literature 5, for example).
(2) Induced Voltages During No-Load De-Energization
However, the above-mentioned conventional improved magnet holding construction has problems of induced voltage during no-load de-energization.
In the conventional improved magnet holding construction, because the magnets are disposed in a vicinity of a surface of the rotor, main magnetic flux or leakage flux from the magnets may have components that cannot be kept inside the rotor and that interlink directly with the stator.
The design is such that magnetic flux leakage levels generate magnetic flux approximately equivalent to one or two volts in an engine idling region at approximately 500 rpm. However, since automotive engines have a variable speed range of approximately 1:10, if, for example, the maximum engine speed is ten times that of idling, the one- or two-volt induced voltages from the magnets may exceed the system voltage of the vehicle and have adverse effects on other on-board equipment. To suppress this, a “reverse field” is required in which the field power source is polarized, and the field current flow is made to flow in reverse at high speeds to weaken the magnetic flux. One problem is that when the direction of flow of the current becomes bidirectional instead of unidirectional, a bidirectional circuit that incorporates an H-bridge is required instead of simple chopper control, increasing the number of components, and raising product costs. Furthermore, unlike a normal field, it is necessary to start this reverse field swiftly in response to increases in engine speed, but since a coil that has a high impedance of several hundred turns is used so as to be able to control the field using a small current of approximately several amperes, it is currently difficult to make the reverse field current flow instantaneously. If the number of field turns is reduced in order to avoid this, new problems arise such as the electric current value of the control power source itself also being increased, increasing control element capacity, and raising product costs.
(3) Demagnetization of Magnets Due to High-Frequency Magnetic Fields Induced by Stator Slots
Having frequency components that are a product of the number of stator slots times rotational frequency per second, slot harmonic magnetic flux is a high-frequency magnetic field of two to three kilohertz. Under such conditions, if the magnets are held between the claw-shaped magnetic poles, or if U-shaped magnets are fitted onto and held by the tip ends of the claw-shaped magnetic poles, portions of the magnets or magnet holding metal fittings are exposed on the rotor surface facing the stator. These exposed magnets or magnet holding metal fittings are heated by induction by the high-frequency magnetic field due to slot harmonics. One problem is that if even a portion of a magnet is heated by induction and reaches a high temperature locally, heat will transfer to the entire magnet, and the magnet will be thermally demagnetized.
Portions of the magnets or magnet holding metal fittings are also exposed on the rotor surface facing the stator in the conventional improved magnet holding construction, making thermal demagnetization of the magnets similarly problematic.
Patent Literature 1: Japanese Patent Laid-Open No. SHO 61-85045 (Gazette)
Patent Literature 2: U.S. Pat. No. 4,959,577 (Specification)
Patent Literature 3: U.S. Pat. No. 5,543,676 (Specification)
Patent Literature 4: Japanese Patent Laid-Open No. 2002-136004 (Gazette)
Patent Literature 5: Japanese Patent Laid-Open No. 2004-153994 (Gazette)